Many electronic circuits benefit from advantages of differential transistor pairs in implementing various amplifier, driver, mixer, or buffer circuits. A basic differential pair may comprise two transistors, for example, bipolar or MOS transistors. In a typical differential pair, the bases or gates of the transistors may form the input nodes and the collectors or drains of the transistors may form the output nodes of the differential pair. The emitters or sources of the transistors may be coupled through a tail circuit that may include a resistor, an inductor, a current source, or a combination of thereof to the ground potential or a negative power supply voltage. Biasing of the differential pair is normally provided via the tail circuit.
The current source may in turn be formed by one or more differential pairs that are biased by stable reference sources such as current sources which are adequately independent from temperature variations. The stability and operating frequency bandwidth of a circuit using the differential pair that employs the current source or the current source itself may be significantly affected by the stability and the bandwidth of the current source.